Does a wife uncovering her face during 'Umrah necessitate expiation or invalidate the 'Umrah?

It is not permissible for a woman in ihram to wear the niqab, as the Prophet (peace be upon him) said: "The woman in ihram should not wear the niqab, nor should she wear gloves." However, she must cover her face with something other than the niqab, such as a sidl or a tarha. Aishah (may Allah be pleased with her) said: "Riders used to pass by us while we were with the Messenger of Allah (peace be upon him) in a state of ihram. When they came abreast of us, one of us would draw her jilbab from her head over her face, and when they had passed us, we would uncover it." The Umrah is valid and is not affected by uncovering the face due to ignorance of the legal ruling.